Traumatic joint injuries are incurred by many Veterans and civilians, and frequently lead to the development of post-traumatic osteoarthritis (OA) years later. At present, there are no interventions that can prevent the development of OA following traumatic joint injury, while interventions used to treat OA focus on pain control and joint replacement, all of which target the symptoms of advanced disease rather than the mechanisms underlying the disease. Although OA has long been viewed as the result of wear and tear, accumulating evidence suggests that inflammatory responses contribute to the pathogenesis of OA. We have identified the inflammatory complement system as a key player in OA. We found that the complement cascade is hyperactivated in OA synovial joints, and that this local hyperactivation may result both from dysregulation of complement-encoding genes in synovial tissues and from activation of complement by cartilage-breakdown products. In the medial meniscectomy (MM) mouse model of OA, mice deficient in the central complement component C5 or in a component of the complement effector MAC (membrane attack complex) were found to be protected against the development of OA-like joint pathology and gait dysfunction. Treatment of wild-type mice with a neutralizing anti-C5 antibody conferred protection, too. In contrast, mice deficient in C4, a component of the classical complement pathway, developed more severe OA-like pathology. We now propose to use two mouse models of post-traumatic OA, as well as biological samples from post-traumatic OA patients and healthy individuals, to further dissect the role of complement in post-traumatic OA. The destabilization of the medial meniscus (DMM) and the MM models of post-traumatic OA will be surgically induced in mice genetically deficient in (i) central complement components common to all three complement pathways; (ii) complement components of the alternative pathway; (iii) complement components of the classical pathway; or (iv) complement components or inhibitors of the MAC effector pathway. We will assess protection from or exacerbation of OA-like pathology by monitoring functional disturbances (as assessed by objective gait analysis) in these mice and evaluating cartilage degradation in mouse joints (by histologic analysis). To determine whether MAC deposition in OA joint tissues is an aberration specific to post-traumatic OA, we will perform immunohistochemical analysis of cartilage and synovial tissue from patients with OA of different duration and causation, as well as from injured but otherwise healthy individuals. The mechanisms of MAC-mediated pathogenicity and of classical-complement-mediated protection will be investigated in a series of in situ and in vitro experiments aimed at determining whether MAC acts to induce the production of inflammatory and degradative mediators by joint tissues and whether classical complement facilitates the phagocytic clearance of inflammatory cartilage debris. How complement is activated in OA will be examined by combining fractionation and proteomic survey of OA synovial fluids with in vitro complement activation assays. Finally, we will test the ability of complement inhibitors (that are in clinical development or FDA-approved) to prevent and treat the development of post-traumatic OA in mice. Success of this proposal could lead to the development of disease-modifying therapeutic interventions for post-traumatic OA.